1. Field of the Invention
The present invention relates to a vacuum regulating valve which is connected, for example, between a vacuum chamber and a vacuum pump to control flow rate so that the pressure in the vacuum chamber is gradually reduced to be a vacuum pressure.
2. Description of the Related Art
For example, the internal pressure of a vacuum chamber of an apparatus for processing a semiconductor wafer or the like may be lowered to a vacuum pressure by using a vacuum pump. In such a procedure, if the gas in the vacuum chamber at atmospheric pressure or high pressure is suddenly evacuated, a large amount of the gas temporarily flows. As a result, some gas turbulence may be generated in the vacuum chamber, and some produced material or the like on the inner wall surface of the vacuum chamber are peeled off. Then, the produced material may be adhered to the semiconductor wafer or the like, or scattered in fluid passages.
In view of the above, a vacuum regulating valve is adopted to avoid a sudden flow rate change. In the vacuum regulating valve, a valve plug is provided in a passage between a vacuum chamber and a vacuum pump for opening/closing the passage by the action of the pressure fluid. A tapered section provided on the valve plug is used to control the flow rate of the fluid flowing through the flow passage (see, for example. Japanese Laid-Open Patent Publication No. 10-252942).
As shown in FIGS. 8 and 9, the conventional vacuum regulating valve 1 comprises a valve seat 4 formed in a flow passage in a body 2 between a first port 3a connected to an unillustrated vacuum chamber and a second port 3b connected to an unillustrated vacuum pump, and a main valve body 5 for opening/closing the passage between the first port 3a and the second port 3b. The main valve body 5 includes a first valve plug 7 connected to a shaft 6, and a second valve plug 9 connected to the lower surface of the first valve plug 7 by an attachment screw 8. The main valve body 5 is seated on or separated from the valve seat 4 by displacing the shaft 6 connected to the main valve body 5 in the axial direction under the action of the pressure fluid.
The valve seat 4 is formed on the upper surface of the body 2 on which the lower surface of the first valve plug 7 is seated. A straight surface 10 and a tapered surface 11 are formed on the outer circumferential surface of the second valve plug 9. The straight surface 10 extends by a predetermined length substantially in parallel to the axis of the second valve plug 9. The tapered surface 11 is diametrally reduced while being inclined by a predetermined angle in a direction toward the opening of the first port 3a of the valve seat 4 from the end position of the straight surface 10. The straight surface 10 abuts against the inner circumferential surface 12 of the first port 3a. 
Further, a lower portion of the first valve plug 7 is formed such that the diameter is expanded radially outwardly. An annular O-ring 13 is installed between the first valve plug 7 and the outer circumferential surface of the second valve plug 9. The air-tightness is retained by the O-ring 13 when the main valve body 5 is seated on the valve seat 4.
In the conventional vacuum regulating valve 1, for example, when the second valve plug 9 of the main valve body 5 is periodically maintained or exchanged, the second valve plug 9 cannot be taken out through the first port 3a to the outside of the body 2 even by detaching the attachment screw 8 in order to release the second valve plug 9 from the first valve plug 7. This is because the second valve plug 9 adjacent to the O-ring 13 has the maximum outer diameter X which is larger than the inner diameter Y of the first port 3a. 
Therefore, to take the second valve plug 9 out of the body 2, a complicated operation is necessary such that the entire main valve body 5 including the second valve plug 9 is taken out through an upper portion of the body 2 and then the main valve body 5 is detached from the second valve plug 9. This operation is also extremely time-consuming. Therefore, operation efficiency is unsatisfactory when the main valve body 5 is maintained.